The use of composite materials such as graphite fiber reinforced epoxies in the fabrication of skins and stringers in aircraft wing structures presents a number of problems in relation to providing lightning strike protection. These problems are different from the problems associated with providing lightning strike protection in conventional aluminum structures because such materials have different electrical properties from aluminum. Such materials are conductive but are considerably less conductive than metals like aluminum and so offer much more resistance to current flow than metallic structures and/or fasteners. Therefore, lightning is attracted to external metal fasteners which attempt to transfer current from a lightning strike into the composite material skin and/or substructure. This can result in internal arcing within the wing structure. Such arcing is undesirable particularly when the wing structure or adjacent cavities, such as the leading or trailing edges, contain fuel and/or combustible vapors.
Another problem is that lightning strikes on such composite material skins can cause burning through such skins. This burn-through can be partially dealt with by making the skins sufficiently thick to confine the damage to repairable surface damage. Further protection may be obtained by providing the outer surface of the skin with various coatings. There have been a number of proposals for providing aircraft structures with lightning protection by providing an outer layer or coating of conductive and/or dielectric material.
U.S. Pat. Nos. 2,982,494, granted May 2, 1961, to M. P. Amason, and 3,416,027, granted Dec. 10, 1968, to M. P. Amason et al, disclose lightning protection systems for aircraft radomes. Each such system includes metallic strips on the exterior surface of the radome for controlling the path of current resulting from a lightning strike.
U.S. Pat. Nos. 3,755,713, granted Aug. 28, 1973, to J. B. Paszkowski; 3,906,308, granted Sept. 16, 1975, to Amason et al; 3,989,984, granted Nov. 2, 1976, to Amason et al; and 4,352,142, granted Sept. 28, 1982, to G. O. Olson, each disclose means for protecting a composite material aircraft structure from lightning. Paszkowski discloses a knitted wire mesh cover over a composite material panel or aerodynamic surface to provide a conductive surface for the aircraft. Amason et al U.S. Pat. No. 3,906,308 disclose a dielectric coating over critical components of the structure. For large span components, spaced metallic strips are affixed to the dielectric outer surface to provide dwell points for the ligthning current channel. Amason et al U.S. Pat. No. 3,989,984 disclose an outer grounded perforated metal layer on the aircraft structure with a bonded dielectric layer beneath such metal layer. At joints in the skin of the structure, exterior surfaces of metallic fasteners are exposed and the conductivity of the fasteners is enhanced by providing the fasteners with suitable coatings. Olson describes a lightning protection system in which the skin of an aircraft structure has an external surface with a sandwich structure consisting of an outer layer of aluminum foil, an intermediate layer of dielectric material, and an inner layer of aluminum foil.
Lightning protection systems for composite material aircraft structures that act to confine a lightning strike to an external portion of the skin present a problem because arcing is likely to occur between the external skin and a metallic fastener or some other metallic member, even when such fastener or member is relatively remote from the initial strike area. When the structure or adjacent cavities contain fuel and/or combustible vapors, such arcing can cause highly dangerous explosions that could lead to destruction of the aircraft.
U.S. Pat. Nos. 2,366,274, granted Jan. 2, 1945, to Luth et al; 3,296,048, granted Jan. 3, 1967, to M. W. Wolfe; and 3,495,494, granted Feb. 17, 1970, to A. L. Scott, each disclose a plastic fastener. Luth et al disclose thermoplastic rivets for fastening together two pieces made from a thermoplastic material. Wolfe discloses a polyurethane screw-like plug for repairing automobile tires. Scott discloses a plastic fastener with reinforced threads. The threads are made from a resin impregnated with glass fiber filaments.
The following United States Patents each disclose a fastener having metallic portions that are electrically insulated:
U.S. Pat. No. 1,674,258, granted June 19, 1928, to H. F. Obergfell et al;
U.S. Pat. No. 2,432,986, granted Dec. 23, 1947, to B. G. Forman;
U.S. Pat. No. No. 2,592,130, granted Apr. 8, 1952, to G. H. Erb et al;
U.S. Pat. No. 2,865,609, granted Dec. 23, 1958, to T. O. Steiner;
U.S. Pat. No. 2,935,552, granted May 3, 1960, to B. G. Forman;
U.S. Pat. No. 2,993,950, granted July 25, 1961, to B. G. Forman;
U.S. Pat. No. 3,086,072 granted Apr. 16, 1963, to B. G. Forman;
U.S. Pat. No. 3,568,311, granted Mar. 9, 1971, to E. J. Lawton; and
U.S. Pat. No. 3,592,100, granted July 13, 1971, to C. Mackiewicz.
Steiner discloses an insulated fastener for electrical fences consisting of a looped wire having a hydrophobic nonconductive sleeve. The other eight patents each disclose a screw-like or bolt-like fastener having a head portion with at least some exposed metal surfaces. Mackiewicz discloses a screw with exposed metal shank and threaded portions and an insulated head. The head has a slot therethrough which exposes the base metal. The screw is designed for use with grouped electrical switches to prevent arcing between adjacent screw heads.
The above patents and the prior art that is discussed and/or cited therein should be studied for the purpose of putting the present invention into proper perspective relative to the prior art.